a large agricultural surplus. although most remained farmers, irrigation freed some 30,000 to 40,000 people to pursue other jobs like making clay figurines. but it was not clay sculpture that transformed teotihuacan into an economic superpower. it was another material -- obsidian, a natural volcanic glass whose edge was as sharp as a razor. obsidian was the knife of the ancient world. 30 miles north of teotihuacan, archaeologists have discovered the major source of obsidian in a place called pachuca. this is alejandro pastna of the mexican institute of anthropology and history, and his colleague rafael cruz. they have come here to map the locations of ancient mines. let's go. [ glass crunching ] obsidian fragments litter the surface, the refuse of thousands of years of mining activity. the glass that formed here by nature was especially prized by the ancients because of a unique quality. interpreter: this obsidian was only formed in this deposit. most obsidian is black or gray, but the green and the golden green i